Commencing in the 1950's, belt drives for dental handpieces were largely replaced by miniature air turbines which operate at relatively high rotational speeds, the turbines for such handpieces being disclosed, for example, in U.S. Pat. No. 3,055,112, issued Sept. 25, 1962, to John V. Borden. Within a relatively few years, such handpieces were widely used and had advanced to the state where most dental handpieces had not only two hoses or tubes to carry air to and from the turbine but also separate water and air hoses to supply water and air separately for cooling the tooth being operated on with the handpiece. Dentists and prior-art workers noted early in the history of the air turbine handpiece that the hose bundle, when pressurized with air and water, acts as if it were alive, applying a varied and unpredictable torque to the handpiece during use. With the turbine driving a drill, the high speed of the drill makes the drill aggressive, fast and free cutting and it is therefore imperative that the dentist using the handpiece have a good and uninterrupted feel of the cutting forces applied at the tooth, else risk cutting away too much of the tooth or, out of caution, cutting too little. The effect of erratic and unpredictable forces, or changes of force, applied to the handpiece by the pressurized hose bundle is a significant disadvantage of handpieces equipped with an air turbine.
Because of the inconvenience and danger caused by the hose bundle of such dental handpieces, prior-art workers have endeavored to provide the handpiece with a multiple path flow swivel, as seen in:
______________________________________ 2,884,695 Ellis 3,173,207 Burzlaff 3,709,630 Pohl et al 3,858,323 Flatland 3,894,338 Loge et al 3,921,296 Harris 4,075,761 Behne et al 4,080,737 Fleer ______________________________________
and U.K. patent application No. GB 2,004,610 filed Apr. 4, 1979. Yet, despite such efforts, air turbine dental handpieces in use today do not have any effective means for swivelling the hose bundle in such a manner as to relieve the handpiece from forces, and particularly changes in forces, applied by the hose bundle. While some swivel attachments are offered, these require such a relatively high torque to operate the swivel that much of the force generated by the hose bundle is simply imparted to the handpiece via the swivel. A number of difficulties appear to have prevented prior-art workers from arriving at a satisfactory solution to the problem. One difficulty is simply the small size of the handpiece. Thus, the outer diameter of the handle of a typical handpiece is 0.62 inch, so that maximum inner diameters are on the order of 0.475 inch, and manufacturing tolerances for the parts of the swivel may be in the millionths of an inch. Next, if cooling air and water are supplied in addition to the air for driving the turbine, and if the exhaust air from the turbine is to be carried away by a hose, the problem of sealing the swivel is difficult, and prior-art approaches have required a plurality of seal elements. When adequate pressures are applied to the seals to prevent leakage, the torque required to turn the swivel becomes excessive. Efforts to solve the seal problem have resulted in manufacturing costs for the swivel so high as to be unrealistic. The problem is further complicated by the fact that the flow of air and water through the swivel tend to cool the parts of the swivel, resulting in dimensional changes which lead to leakage, jamming or erratic operation.